The research will investigate conditional discrimination learning in animal subjects. The principal issue is whether such learning is best characterized as the learning of an "if-then" rule, where one element of a stimulus set serves the instructional function of specifying the associative relations involving the other elements of the set, or as the acquisition of attention to stimulus configuration, which is a stimulus element created whenever stimulus elements are conjoined, but which obeys the functional relations applicable to other simpler stimulus elements (e.g., color). Underlying this question is the issue of whether there is a single type of stimulus control, with a general set of laws, or whether instead there are several types of stimulus control, each corresponding to a different type of associative unit in which the stimulus may participate.